I like both, I think Morten wouldn't be as good as Kjetil on Ashes' songs and Kjetil wouldn't fit on WW's or BTV's songs.
But if I must choose one, i choose Kjetil, he has a very energetic voice. I think that songs like Libre, Circus or The Wretched aren't the same without him (although i know many people will say that anyways they aren't the same without Vibeke).
Just listen to Kjetil's voice on these verses from Circus:
"I am your bagful of fear
I'll commit, I'll admit
I'll be the dagger! DAGGER YEAAAAAHHHHH!!!"
Too bad he left the band :S
BTW i also love Ronny Thorsen's vocals, they fit WOG very well. He's one of the reasons why i'm also a Trail Of Tears fan